e2fsck: clear i_block[] when there are too many bad mappings on a special inode

e2fsck: clear i_block[] when there are too many bad mappings on a special inode

If we decide to clear a special inode because of bad mappings, we need
to zero the i_block array.  The clearing routine depends on setting
i_links_count to zero to keep us from re-checking the block maps,
but that field isn't checked for special inodes.  Therefore, if we
haven't erased the mappings, check_blocks will restart fsck and fsck
will try to check the blocks again, leading to an infinite loop.

(This seems easy to trigger if the bootloader inode extent map is
corrupted.)

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>